Existing cable television systems have recently been used to deliver digital data in addition to analog television broadcasts. The digital data is delivered by modulating it onto a radio frequency carrier using any one of a number of different modulation schemes. The data can be anything such as digitized video, telephone calls, broadband internet access, etc.
Cable Labs is an industry consortium of cable system operators which develops standards for the transmission of digital data over hybrid fiber coaxial cable (HFC) of CATV systems. Cable Labs also certifies equipment such as cable modems and head end transceivers developed by various vendors as either being in compliance or not being within compliance with Cable Labs standards.
Cable Labs has previously developed the DOCSIS 1.0 and 1.1 standards for transmitting digital data over cable. Many cable systems exist which have deployed cable modems which comply with these DOCSIS 1.0 and 1.1 standards. The most complex constellation, i.e., modulation type, which these DOCSIS 1.0 and 1.1 cable modems were designed to receive in the downstream is 256-QAM. QAM stands for Quadrature Amplitude Modulation and encodes bits into constellation points each of which has a phase and a magnitude. The phase or angle of the constellation point from the origin defines the values of certain bits in the vector which defines the constellation point. The amplitude of the vector to that constellation point defines the values of other bits in the vector which defines the constellation point.
As the services which are to be delivered over HFC get more complex and multimedia rich, more data must be sent. There are only two choices to send more data on a radio frequency carrier: raise the symbol rate (the clock rate defining the interval during which each symbol or constellation point is modulated onto the carrier); or use a more complex constellation to send more data during each symbol time but still using the same symbol rate.
Unfortunately, raising the symbol rate is not a good option in most cable systems because raising the symbol rate also raises the bandwidth consumed by the RF signal, and the bandwidth is a limited resource pool.
In QAM modulation, the magnitude and phase information of each symbol are represented by different bits of the symbol. Each of these sets of bits can be translated into an analog value which represents the values of those binary bits in a base 10 numbering system. The analog values for the magnitudes of each symbol are used to amplitude modulate a first radio frequency carrier during different segments of time representing the symbol periods. In other words, during each symbol period, the analog value of the magnitude of a corresponding symbol or constellation point is used to amplitude modulate the RF carrier. Likewise, the analog value of the phase bits of each symbol is used to amplitude modulate a second RF carrier which has the same frequency as the first RF carrier but which is 90 degrees out of phase therewith. Hence the term quadrature amplitude modulation.
When the symbol rate is increased, the spacing between the changes in amplitude becomes smaller. This increases the bandwidth consumed by the signal. If these high frequency components are cut off in the receiver passband filters, the received signal will be distorted in shape. This will cause bit errors in the received signal.
Therefore, a need has arisen for a method and apparatus that allows more data to be sent without raising the symbol rate using a 1024-QAM constellation, and which allows the legacy cable modems which are already deployed and which can only receive at most 256-QAM modulated signals to still be used in the same logical channel with newer modems which can recover data from 1024-QAM constellation points.